An interesting discovery. I'd thought of using the 1/2" copper tubing since the OD is .625". Still thinking of using it.

I'd assumed the ID of tubing was the same as the nominal dimension - 1/2". NOT. The ID of 1/2" tubing is about .570 or so. That just wiped out the idea of using a 50 caliber slug as an insert for the 20ga.

An interesting project, but before you go further, I'd suggest thinking about this: Shotgun barrels get rather thin about 6-8" from the breechface, which well suits the burn rate of shotgun powders. These propellants burn quickly, with the maximum pressures achieved in the first few inches of the barrel then dropping off rapidly. That's why the breech end of a shotgun barrel is much thicker than the muzzle.

The use of slow burning rifle propellants changes things. Now the max pressure is carried further down the tube, and if the near-max pressure reaches the thinner portion of the barrel, the barrel is at risk of bursting...if that pressure is too high. This pressure will certainly be higher than that of shotgun powder. I strongly recommend NOT using slower rifle powders in this arm without safely testing it first. Tie the shotgun to a mounted car tire and fire it with a string. I sure wouldn't want to have a thin shotgun barrel split and throw out powder and gasses so close to my face.... Please think safety, not just experimentation.

The plot thickens.

Just started to mike out some things I had on-hand.

First, 1/2" copper tubing has an OD of .625 which is the exact groove diameter for 20 ga in the NEF.

Second, the connectors for 1/2" measure .630 ID and .703 OD AND the Lyman 12 ga slug measures 0.698 and a round ball cast measures .720".

Third, I found a single cavity mould picked up a couple of years ago at an antique/flee market (iron handles built in) measures 0.623" and weighs 546.4grains hard cast (1.2oz).

Sitting right on top of it for a couple of years. Guess I've got some reloading - maybe paper patching - to do.